The root indeterminacy-to-determinacy developmental switch is operated through a folate-dependent pathway in Arabidopsis thaliana

Folate depletion impact on the cell cycle results in restricted primary root growth in Arabidopsis

Folates are vital one carbon donors and acceptors for a whole range of key biochemical reactions, including the biosynthesis of DNA building blocks. Plants use one carbon metabolism as a jack of all trades in their growth and development. Depletion of folates impedes root growth in Arabidopsis thaliana, but the mechanistic basis behind this function is still obscure. A global transcriptomic study hinted that folate depletion may cause misregulation of cell cycle progression. However, investigations on a direct connection thereof are scarce. We confirmed the effect of methotrexate (MTX), a folate biosynthesis inhibitor, on the expression of cell cycle genes. Subsequently, we determined the effect of MTX on root morphology and cell cycle progression through phase-specific cell cycle reporter analyses. Our study reveals that folate depletion affects the expression of cell cycle regulatory genes in roots, thereby suppressing cell cycle progression. We confirmed, through DNA labelling by EdU, that MTX treatment leads to arrest in the S phase of meristematic cells, likely due to the lack of DNA precursors. Further, we noted an accumulation of the A-type CYCA3;1 cyclin at the root tip, suggesting a possible link with the observed loss of apical dominance. Overall, our study shows that the restricted cell division and cell cycle progression is one of the reasons behind the loss of primary root growth upon folate depletion.

Formate-tetrahydrofolate ligase: supplying the cytosolic one-carbon network in roots with one-carbon units originating from glycolate

The metabolism of tetrahydrofolate (H4PteGlun)-bound one-carbon (C1) units (C1 metabolism) is multifaceted and required for plant growth, but it is unclear what of many possible synthesis pathways provide C1 units in specific organelles and tissues. One possible source of C1 units is via formate-tetrahydrofolate ligase, which catalyzes the reversible ATP-driven production of 10-formyltetrahydrofolate (10-formyl-H4PteGlun) from formate and tetrahydrofolate (H4PteGlun). Here, we report biochemical and functional characterization of the enzyme from Arabidopsis thaliana (AtFTHFL). We show that the recombinant AtFTHFL has lower Km and kcat values with pentaglutamyl tetrahydrofolate (H4PteGlu5) as compared to monoglutamyl tetrahydrofolate (H4PteGlu1), resulting in virtually identical catalytic efficiencies for the two substrates. Stable transformation of Arabidopsis plants with the EGFP-tagged AtFTHFL, followed with fluorescence microscopy, demonstrated cytosolic signal. Two independent T-DNA insertion lines with impaired AtFTHFL function had shorter roots compared to the wild type plants, demonstrating the importance of this enzyme for root growth. Overexpressing AtFTHFL led to the accumulation of H4PteGlun + 5,10-methylene-H4PteGlun and serine, accompanied with the depletion of formate and glycolate, in roots of the transgenic Arabidopsis plants. This metabolic adjustment supports the hypothesis that AtFTHFL feeds the cytosolic C1 network in roots with C1 units originating from glycolate, and that these units are then used mainly for biosynthesis of serine, and not as much for the biosynthesis of 5-methyl-H4PteGlun, methionine, and S-adenosylmethionine. This finding has implications for any future attempts to engineer one-carbon unit-requiring products through manipulation of the one-carbon metabolic network in non-photosynthetic organs.

OsAlR3 regulates aluminum tolerance through promoting the secretion of organic acids and the expression of antioxidant genes in rice

In acidic soils, aluminum (Al) toxicity inhibits the growth and development of plant roots and affects nutrient and water absorption, leading to reduced yield and quality. Therefore, it is crucial to investigate and identify candidate genes for Al tolerance and elucidate their physiological and molecular mechanisms under Al stress. In this study, we identified a new gene OsAlR3 regulating Al tolerance, and analyzed its mechanism from physiological, transcriptional and metabolic levels. Compared with the WT, malondialdehyde (MDA) and hydrogen peroxide (H2O2) content were significantly increased, superoxide dismutase (SOD) activity and citric acid (CA) content were significantly decreased in the osalr3 mutant lines when exposed to Al stress. Under Al stress, the osalr3 exhibited decreased expression of antioxidant-related genes and lower organic acid content compared with WT. Integrated transcriptome and metabolome analysis showed the phenylpropanoid biosynthetic pathway plays an important role in OsAlR3-mediated Al tolerance. Exogenous CA and oxalic acid (OA) could increase total root length and enhance the antioxidant capacity in the mutant lines under Al stress. Conclusively, we found a new gene OsAlR3 that positively regulates Al tolerance by promoting the chelation of Al ions through the secretion of organic acids, and increasing the expression of antioxidant genes.

Not so hidden anymore: Advances and challenges in understanding root growth under water deficits

Limited water availability is a major environmental factor constraining plant development and crop yields. One of the prominent adaptations of plants to water deficits is the maintenance of root growth that enables sustained access to soil water. Despite early recognition of the adaptive significance of root growth maintenance under water deficits, progress in understanding has been hampered by the inherent complexity of root systems and their interactions with the soil environment. We highlight selected milestones in the understanding of root growth responses to water deficits, with emphasis on founding studies that have shaped current knowledge and set the stage for further investigation. We revisit the concept of integrated biophysical and metabolic regulation of plant growth and use this framework to review central growth-regulatory processes occurring within root growth zones under water stress at subcellular to organ scales. Key topics include the primary processes of modifications of cell wall-yielding properties and osmotic adjustment, as well as regulatory roles of abscisic acid and its interactions with other hormones. We include consideration of long-recognized responses for which detailed mechanistic understanding has been elusive until recently, for example hydrotropism, and identify gaps in knowledge, ongoing challenges, and opportunities for future research.

The Roles of Functional Amino Acids in Plant Growth and Development

Plants incorporate acquired carbon and nitrogen into amino acid metabolism, whereby the building blocks of proteins and the precursors of various metabolites are produced. This fundamental demand requires tight amino acid metabolism to sustain physiological homeostasis. There is increasing evidence that amino acid metabolism undergoes plastic alteration to orchestrate specific growth and developmental events. Consequently, there has been a gradual exploration of the interface at which amino acid metabolism and plant morphogenesis are mutually affected. This research progress offers an opportunity to explore amino acid metabolism, with the goal to understand how it can be modulated to serve special cellular needs and regulate specific growth and developmental pathways. Continuous improvements in the sensitivity and coverage of metabolomics technology, along with the development of chemoinformatics, have allowed the investigation of these research questions. In this review, we summarize the roles of threonine, serine, arginine and γ-aminobutyric acid as representative examples of amino acids relevant to specific developmental processes in plants ('functional amino acids'). Our objective is to expand perspectives regarding amino acid metabolism beyond the conventional view that it is merely life-supporting machinery.

Arabidopsis Root Development Regulation by the Endogenous Folate Precursor, Para-Aminobenzoic Acid, via Modulation of the Root Cell Cycle

The continuous growth of roots depends on their ability to maintain a balanced ratio between cell production and cell differentiation at the tip. This process is regulated by the hormonal balance of cytokinin and auxin. However, other important regulators, such as plant folates, also play a regulatory role. In this study, we investigated the impact of the folate precursor para-aminobenzoic acid (PABA) on root development. Using pharmacological, genetic, and imaging approaches, we show that the growth of Arabidopsis thaliana roots is repressed by either supplementing the growth medium with PABA or overexpressing the PABA synthesis gene GAT-ADCS. This is associated with a smaller root meristem consisting of fewer cells. Conversely, reducing the levels of free root endogenous PABA results in longer roots with extended meristems. We provide evidence that PABA represses Arabidopsis root growth in a folate-independent manner and likely acts through two mechanisms: (i) the G2/M transition of cell division in the root apical meristem and (ii) promoting premature cell differentiation in the transition zone. These data collectively suggest that PABA plays a role in Arabidopsis root growth at the intersection between cell division and cell differentiation.

How metabolism and development are intertwined in space and time

Developmental transitions, occurring throughout the life cycle of plants, require precise regulation of metabolic processes to generate the energy and resources necessary for the committed growth processes. In parallel, the establishment of new cells, tissues, and even organs, alongside their differentiation provoke profound changes in metabolism. It is increasingly being recognized that there is a certain degree of feedback regulation between the components and products of metabolic pathways and developmental regulators. The generation of large-scale metabolomics datasets during developmental transitions, in combination with molecular genetic approaches has helped to further our knowledge on the functional importance of metabolic regulation of development. In this perspective article, we provide insights into studies that elucidate interactions between metabolism and development at the temporal and spatial scales. We additionally discuss how this influences cell growth-related processes. We also highlight how metabolic intermediates function as signaling molecules to direct plant development in response to changing internal and external conditions.

Interplay between the Brassica napus phytoglobin (BnPgb1), folic acid, and antioxidant responses enhances plant tolerance to waterlogging

Oxygen deprivation by waterlogging reduces the productivity of several crop species, including the oil-producing crop Brassica napus L., which is highly sensitive to excess moisture. Among factors induced by oxygen deficiency are phytoglobins (Pgbs), heme-containing proteins known to ameliorate the response of plants to the stress. This study examined the early responses to waterlogging in B. napus plants over-expressing or down-regulating the class 1 (BnPgb1) and class 2 (BnPgb2) Pgbs. The depression of gas exchange parameters and plant biomass was exacerbated by the suppression of BnPgb1, while suppression of BnPgb2 did not evoke any changes. This suggests that natural occurring levels of BnPgb1 (but not BnPg2) are required for the response of the plants to waterlogging. Typical waterlogging symptoms, including the accumulation of reactive oxygen species (ROS) and the deterioration of the root apical meristem (RAM) were attenuated by over-expression of BnPgb1. These effects were associated with the activation of antioxidant system and the transcriptional induction of folic acid (FA). Pharmacological treatments revealed that high levels of FA were sufficient to revert the inhibitory effect of waterlogging, suggesting that the interplay between BnPgb1, antioxidant responses and FA might contribute to plant tolerance to waterlogging stress.

Folate shapes plant root architecture by affecting auxin distribution

Folate (vitamin B9) is important for plant root development, but the mechanism is largely unknown. Here we characterized a root defective mutant, folb2, in Arabidopsis, which has severe developmental defects in the primary root. The root apical meristem of the folb2 mutant is impaired, and adventitious roots are frequently found at the root-hypocotyl junction. Positional cloning revealed that a 61-bp deletion is present in the predicted junction region of the promoter and the 5' untranslated region of AtFolB2, a gene encoding a dihydroneopterin aldolase that functions in folate biosynthesis. This mutation leads to a significant reduction in the transcript level of AtFolB2. Liquid chromatography-mass spectrometry analysis showed that the contents of the selected folate compounds were decreased in folb2. Arabidopsis AtFolB2 knockdown lines phenocopy the folb2 mutant. On the other hand, the application of exogenous 5-formyltetrahydrofolic acid could rescue the root phenotype of folb2, indicating that the root phenotype is indeed related to the folate level. Further analysis revealed that folate could promote rootward auxin transport through auxin transporters and that folate may affect particular auxin/indole-3-acetic acid proteins and auxin response factors. Our findings provide new insights into the important role of folic acid in shaping root structure.

A mutation in THREONINE SYNTHASE 1 uncouples proliferation and transition domains of the root apical meristem: experimental evidence and in silico proposed mechanism

A continuum from stem to transit-amplifying to a differentiated cell state is a common theme in multicellular organisms. In the plant root apical meristem (RAM), transit-amplifying cells are organized into two domains: cells from the proliferation domain (PD) are displaced to the transition domain (TD), suggesting that both domains are necessarily coupled. Here, we show that in the Arabidopsis thaliana mto2-2 mutant, in which threonine (Thr) synthesis is affected, the RAM lacks the PD. Through a combination of cell length profile analysis, mathematical modeling and molecular markers, we establish that the PD and TD can be uncoupled. Remarkably, although the RAM of mto2-2 is represented solely by the TD, the known factors of RAM maintenance and auxin signaling are expressed in the mutant. Mathematical modeling predicts that the stem cell niche depends on Thr metabolism and that, when disturbed, the normal continuum of cell states becomes aborted.

Targeted and Untargeted Metabolomic Analyses Reveal Organ Specificity of Specialized Metabolites in the Model Grass Brachypodium distachyon

Brachypodium distachyon, because of its fully sequenced genome, is frequently used as a model grass species. However, its metabolome, which constitutes an indispensable element of complex biological systems, remains poorly characterized. In this study, we conducted comprehensive, liquid chromatography-mass spectrometry (LC-MS)-based metabolomic examination of roots, leaves and spikes of Brachypodium Bd21 and Bd3-1 lines. Our pathway enrichment analysis emphasised the accumulation of specialized metabolites representing the flavonoid biosynthetic pathway in parallel with processes related to nucleotide, sugar and amino acid metabolism. Similarities in metabolite profiles between both lines were relatively high in roots and leaves while spikes showed higher metabolic variance within both accessions. In roots, differences between Bd21 and Bd3-1 lines were manifested primarily in diterpenoid metabolism, while differences within spikes and leaves concerned nucleotide metabolism and nitrogen management. Additionally, sulphate-containing metabolites differentiated Bd21 and Bd3-1 lines in spikes. Structural analysis based on MS fragmentation spectra enabled identification of 93 specialized metabolites. Among them phenylpropanoids and flavonoids derivatives were mainly determined. As compared with closely related barley and wheat species, metabolic profile of Brachypodium is characterized with presence of threonate derivatives of hydroxycinnamic acids.

Galactose induces formation of cell wall stubs and cell death in Arabidopsis roots

Arabidopsis seedlings growing on low concentration of galactose stop regular root growth. Incomplete cell division with cell wall stubs, binuclear and giant cells and lignified root tips are observed. Galactose is a sugar abundant in root cell walls of Arabidopsis. Nevertheless, we found that the germination of Arabidopsis seedlings on galactose containing media causes a strong modification of the root development, as shown by analysing the root with microscopy methods ranging from the bright field over confocal to transmission electron microscopy. At concentrations of about 1 mM, the growth of the primary root stops after a few days though stem cell markers like WOX5 are still expressed. The root tip swells and forms a slightly opaque, partially lignified structure in parts of the cortex and the central cylinder. The formation of the cell plate after mitosis is impaired, often leading to cell wall stubs and binuclear cells. Some cells in the cortex and the central cylinder degenerate, while some rhizodermal and cortical cells increase massively in size. The galactose toxicity phenotype in Arabidopsis depends on the activity of galactokinase and is completely diminished in galactokinase knock-out lines. From the comparison of the galactose toxicity phenotype with those of cytokinesis mutants and plants treated with appropriate inhibitors we speculate that the toxicity syndrome of galactose is caused by interference with intracellular vesicle transport or cell wall biogenesis.

Live Plant Cell Tracking: Fiji plugin to analyze cell proliferation dynamics and understand morphogenesis

Arabidopsis (Arabidopsis thaliana) primary and lateral roots (LRs) are well suited for 3D and 4D microscopy, and their development provides an ideal system for studying morphogenesis and cell proliferation dynamics. With fast-advancing microscopy techniques used for live-imaging, whole tissue data are increasingly available, yet present the great challenge of analyzing complex interactions within cell populations. We developed a plugin "Live Plant Cell Tracking" (LiPlaCeT) coupled to the publicly available ImageJ image analysis program and generated a pipeline that allows, with the aid of LiPlaCeT, 4D cell tracking and lineage analysis of populations of dividing and growing cells. The LiPlaCeT plugin contains ad hoc ergonomic curating tools, making it very simple to use for manual cell tracking, especially when the signal-to-noise ratio of images is low or variable in time or 3D space and when automated methods may fail. Performing time-lapse experiments and using cell-tracking data extracted with the assistance of LiPlaCeT, we accomplished deep analyses of cell proliferation and clonal relations in the whole developing LR primordia and constructed genealogical trees. We also used cell-tracking data for endodermis cells of the root apical meristem (RAM) and performed automated analyses of cell population dynamics using ParaView software (also publicly available). Using the RAM as an example, we also showed how LiPlaCeT can be used to generate information at the whole-tissue level regarding cell length, cell position, cell growth rate, cell displacement rate, and proliferation activity. The pipeline will be useful in live-imaging studies of roots and other plant organs to understand complex interactions within proliferating and growing cell populations. The plugin includes a step-by-step user manual and a dataset example that are available at https://www.ibt.unam.mx/documentos/diversos/LiPlaCeT.zip.

Interkingdom Comparison of Threonine Metabolism for Stem Cell Maintenance in Plants and Animals

In multicellular organisms, tissue generation, maintenance, and homeostasis depend on stem cells. Cellular metabolic status is an essential component of different differentiated states, from stem to fully differentiated cells. Threonine (Thr) metabolism has emerged as a critical factor required to maintain pluripotent/multipotent stem cells in both plants and animals. Thus, both kingdoms conserved or converged upon this fundamental feature of stem cell function. Here, we examine similarities and differences in Thr metabolism-dependent mechanisms supporting stem cell maintenance in these two kingdoms. We then consider common features of Thr metabolism in stem cell maintenance and predict and speculate that some knowledge about Thr metabolism and its role in stem cell function in one kingdom may apply to the other. Finally, we outline future research directions to explore these hypotheses.

Hormonal Regulation of Stem Cell Proliferation at the Arabidopsis thaliana Root Stem Cell Niche

The root stem cell niche (SCN) of Arabidopsis thaliana consists of the quiescent center (QC) cells and the surrounding initial stem cells that produce progeny to replenish all the tissues of the root. The QC cells divide rather slowly relative to the initials, yet most root tissues can be formed from these cells, depending on the requirements of the plant. Hormones are fundamental cues that link such needs with the cell proliferation and differentiation dynamics at the root SCN. Nonetheless, the crosstalk between hormone signaling and the mechanisms that regulate developmental adjustments is still not fully understood. Developmental transcriptional regulatory networks modulate hormone biosynthesis, metabolism, and signaling, and conversely, hormonal responses can affect the expression of transcription factors involved in the spatiotemporal patterning at the root SCN. Hence, a complex genetic-hormonal regulatory network underlies root patterning, growth, and plasticity in response to changing environmental conditions. In this review, we summarize the scientific literature regarding the role of hormones in the regulation of QC cell proliferation and discuss how hormonal signaling pathways may be integrated with the gene regulatory network that underlies cell fate in the root SCN. The conceptual framework we present aims to contribute to the understanding of the mechanisms by which hormonal pathways act as integrators of environmental cues to impact on SCN activity.

Influence of Sulfonamide Contamination Derived from Veterinary Antibiotics on Plant Growth and Development

Veterinary antibiotics such as sulfonamides are widely used to increase feed efficiency and to protect against disease in livestock production. The sulfonamide antimicrobial mechanism involves the blocking of folate biosynthesis by inhibiting bacterial dihydropteroate synthase (DHPS) activity competitively. Interestingly, most treatment antibiotics can be released into the environment via manure and result in significant diffuse pollution in the environment. However, the physiological effects of sulfonamide during plant growth and development remain elusive because the plant response is dependent on folate biosynthesis and the concentration of antibiotics. Here, we present a chemical interaction docking model between Napa cabbage (Brassica campestris) DHPS and sulfamethoxazole and sulfamethazine, which are the most abundant sulfonamides detected in the environment. Furthermore, seedling growth inhibition was observed in lentil bean (Lens culinaris), rice (Oryza sativa), and Napa cabbage plants upon sulfonamide exposure. The results revealed that sulfonamide antibiotics target plant DHPS in a module similar to bacterial DHPS and affect early growth and the development of crop seedlings. Taking these results together, we suggest that sulfonamides act as pollutants in crop fields.

Determinate root development in the halted primary root1 mutant of Arabidopsis correlates with death of root initial cells and an enhanced auxin response

The transit from indeterminate to determinate root developmental program compromises growth and causes the differentiation of the meristem, but a direct link between this process with auxin signaling and/or viability of initial cells remains untested. Here, through the isolation and characterization of the halted primary root1 (hpr1) mutant of Arabidopsis, which develops primary and lateral roots with genetically stable determinate growth after germination, we show that the differentiation of the root meristem correlates with enhanced auxin responsiveness and is preceded by the death of provasculature initial cells in both primary and lateral roots. Supplementation of indole-3-acetic acid causes both a dose-dependent repression of primary root growth and an induction of DR5:uidA expression in wild-type seedlings, and these effects were exacerbated in hpr1 mutants. The damage of provasculature initial cells in the root of hpr1 mutants occurred at earlier times than the full differentiation of the meristem, and correlates with a reduced expression domain of CycB1:uidA and PRZ:uidA. Thus, HPR1 plays critical functions for stem cell maintenance, auxin homeostasis, cell division in the meristem, and indeterminate root growth.

Root stem cell niche maintenance and apical meristem activity critically depend on THREONINE SYNTHASE1

Indeterminate root growth depends on the stem cell niche (SCN) and root apical meristem (RAM) maintenance whose regulation permits plasticity in root system formation. Using a forward genetics approach, we isolated the moots koom1 ('short root' in Mayan) mutant that shows complete primary RAM exhaustion and abolished SCN activity. We identified that this phenotype is caused by a point mutation in the METHIONINE OVERACCUMULATOR2 (MTO2) gene that encodes THREONINE SYNTHASE1 and renamed the mutant as mto2-2. The amino acid profile showed drastic changes, most notorious of which was accumulation of methionine. In non-allelic mto1-1 (Arabidopsis thaliana cystathionine gamma-synthetase1) and mto3-1 (S-adenosylmethionine synthetase) mutants, both with an increased methionine level, the RAM size was similar to that of the wild type, suggesting that methionine overaccumulation itself did not cause RAM exhaustion in mto2 mutants. When mto2-2 RAM is not yet completely exhausted, exogenous threonine induced de novo SCN establishment and root growth recovery. The threonine-dependent RAM re-establishment in mto2-2 suggests that threonine is a limiting factor for RAM maintenance. In the root, MTO2 was predominantly expressed in the RAM. The essential role of threonine in mouse embryonic stem cells and in RAM maintenance suggests that common regulatory mechanisms may operate in plant and animal SCN maintenance.

Lateral Root Primordium Morphogenesis in Angiosperms

Morphogenetic processes are the basis of new organ formation. Lateral roots (LRs) are the building blocks of the root system. After LR initiation and before LR emergence, a new lateral root primordium (LRP) forms. During this period, the organization and functionality of the prospective LR is defined. Thus, proper LRP morphogenesis is a decisive process during root system formation. Most current studies on LRP morphogenesis have been performed in the model species Arabidopsis thaliana; little is known about this process in other angiosperms. To understand LRP morphogenesis from a wider perspective, we review both contemporary and earlier studies. The latter are largely forgotten, and we attempted to integrate them into present-day research. In particular, we consider in detail the participation of parent root tissue in LRP formation, cell proliferation and timing during LRP morphogenesis, and the hormonal and genetic regulation of LRP morphogenesis. Cell type identity acquisition and new stem cell establishement during LRP morphogenesis are also considered. Within each of these facets, unanswered or poorly understood questions are identified to help define future research in the field. Finally, we discuss emerging research avenues and new technologies that could be used to answer the remaining questions in studies of LRP morphogenesis.

Toward Eradication of B-Vitamin Deficiencies: Considerations for Crop Biofortification

'Hidden hunger' involves insufficient intake of micronutrients and is estimated to affect over two billion people on a global scale. Malnutrition of vitamins and minerals is known to cause an alarming number of casualties, even in the developed world. Many staple crops, although serving as the main dietary component for large population groups, deliver inadequate amounts of micronutrients. Biofortification, the augmentation of natural micronutrient levels in crop products through breeding or genetic engineering, is a pivotal tool in the fight against micronutrient malnutrition (MNM). Although these approaches have shown to be successful in several species, a more extensive knowledge of plant metabolism and function of these micronutrients is required to refine and improve biofortification strategies. This review focuses on the relevant B-vitamins (B1, B6, and B9). First, the role of these vitamins in plant physiology is elaborated, as well their biosynthesis. Second, the rationale behind vitamin biofortification is illustrated in view of pathophysiology and epidemiology of the deficiency. Furthermore, advances in biofortification, via metabolic engineering or breeding, are presented. Finally, considerations on B-vitamin multi-biofortified crops are raised, comprising the possible interplay of these vitamins in planta.

Folic acid orchestrates root development linking cell elongation with auxin response and acts independently of the TARGET OF RAPAMYCIN signaling in Arabidopsis thaliana

Folic acid is a precursor of tetrahydrofolate (vitamin B9), which is an essential cofactor in most organisms, acting as a carrier for one-carbon units in enzymatic reactions. In this work, we employed pharmacological, genetic and confocal imaging strategies to unravel the signaling mechanism by which folic acid modulates root growth and development. Folic acid supplementation inhibits primary root elongation and induces lateral root formation in a concentration-dependent manner. An analysis of the expression of cell cycle genes pCycD6;1:GFP and CycB1:uidA, and cell expansion Exp7:uidA showed that folic acid promotes cell division but prevented cell elongation, and this correlated with altered expression of auxin-responsive DR5:GFP gene, and PIN1:PIN1:GFP, PIN3:PIN3:GFP, and PIN7:PIN7:GFP auxin transporters at the columella and vasculature of primary roots, whereas mutants defective in auxin signaling (tir1/afb1/afb2 [receptors], slr1 [repressor] and arf7/arf19 [transcription factors]) were less sensitive to folic acid induced primary root shortening and lateral root proliferation. Comparison of growth of WT and TARGET OF RAPAMYCIN (TOR) antisense lines indicates that folic acid acts by an alternative mechanism to this central regulator. Thus, folic acid modulation of root architecture involves auxin and acts independently of the TOR kinase to influence basic cellular programs.

The MEDIATOR genes MED12 and MED13 control Arabidopsis root system configuration influencing sugar and auxin responses

Arabidopsis med12 and med13 mutants exhibit shoot and root phenotypes related to an altered auxin homeostasis. Sucrose supplementation reactivates both cell division and elongation in primary roots as well as auxin-responsive and stem cell niche gene expression in these mutants. An analysis of primary root growth of WT, med12, aux1-7 and med12 aux1 single and double mutants in response to sucrose and/or N-1-naphthylphthalamic acid (NPA) placed MED12 upstream of auxin transport for the sugar modulation of root growth. The MEDIATOR (MED) complex plays diverse functions in plant development, hormone signaling and biotic and abiotic stress tolerance through coordination of transcription. Here, we performed genetic, developmental, molecular and pharmacological analyses to characterize the role of MED12 and MED13 on the configuration of root architecture and its relationship with auxin and sugar responses. Arabidopsis med12 and med13 single mutants exhibit shoot and root phenotypes consistent with altered auxin homeostasis including altered primary root growth, lateral root development, and root hair elongation. MED12 and MED13 were required for activation of cell division and elongation in primary roots, as well as auxin-responsive and stem cell niche gene expression. Remarkably, most of these mutant phenotypes were rescued by supplying sucrose to the growth medium. The growth response of primary roots of WT, med12, aux1-7 and med12 aux1 single and double mutants to sucrose and application of auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) revealed the correlation of med12 phenotype with the activity of the auxin intake permease and suggests that MED12 acts upstream of AUX1 in the root growth response to sugar. These data provide compelling evidence that MEDIATOR links sugar sensing to auxin transport and distribution during root morphogenesis.

Plastidial Folate Prevents Starch Biosynthesis Triggered by Sugar Influx into Non-Photosynthetic Plastids of Arabidopsis

Regulation of sucrose-starch interconversion in plants is important to maintain energy supplies necessary for viability and growth. Arabidopsis mutants were screened for aberrant responses to sucrose to identify candidates with a defect in the regulation of starch biosynthesis. One such mutant, fpgs1-4, accumulated substantial amounts of starch in non-photosynthetic cells. Dark-grown mutant seedlings exhibited shortened hypocotyls and accumulated starch in etioplasts when supplied with exogenous sucrose/glucose. Similar starch accumulation from exogenous sucrose was observed in mutant chloroplasts, when photosynthesis was prevented by organ culture in darkness. Molecular genetic analyses revealed that the mutant was defective in plastidial folylpolyglutamate synthetase, one of the enzymes engaged in folate biosynthesis. Active folate derivatives are important biomolecules that function as cofactors for a variety of enzymes. Exogenously supplied 5-formyl-tetrahydrofolate abrogated the mutant phenotypes, indicating that the fpgs1-4 mutant produced insufficient folate derivative levels. In addition, the antifolate agents methotrexate and 5-fluorouracil induced starch accumulation from exogenously supplied sucrose in dark-grown seedlings of wild-type Arabidopsis. These results indicate that plastidial folate suppresses starch biosynthesis triggered by sugar influx into non-photosynthetic cells, demonstrating a hitherto unsuspected link between plastidial folate and starch metabolism.

Casparian bands and suberin lamellae in exodermis of lateral roots: an important trait of roots system response to abiotic stress factors

Background and Aims

Root absorptive characteristics rely on the presence of apoplastic barriers. However, little is known about the establishment of these barriers within a complex root system, particularly in a major portion of them - the lateral roots. In Zea mays L., the exodermis differentiates under the influence of growth conditions. Therefore, the species presents a suitable model to elucidate the cross-talk among environmental conditions, branching pattern and the maturation of barriers within a complex root system involved in the definition of the plant-soil interface. The study describes the extent to which lateral roots differentiate apoplastic barriers in response to changeable environmental conditions.

Methods

The branching, permeability of the outer cell layers and differentiation of the endo- and exodermis were studied in primary roots and various laterals under different types of stress of agronomic importance (salinity, heavy metal toxicity, hypoxia, etc.). Histochemical methods, image analysis and apoplastic tracer assays were utilized.

Key Results

The results show that the impact of growth conditions on the differentiation of both the endodermis and exodermis is modulated according to the type/diameter of the root. Fine laterals clearly represent that portion of a complex root system with a less advanced state of barrier differentiation, but with substantial ability to modify exodermis differentiation in response to environmental conditions. In addition, some degree of autonomy in exodermal establishment of Casparian bands (CBs) vs. suberin lamellae (SLs) was observed, as the absence of lignified exodermal CBs did not always fit with the lack of SLs.

Conclusions

This study highlights the importance of lateral roots, and provides a first look into the developmental variations of apoplastic barriers within a complex root system. It emphasizes that branching and differentiation of barriers in fine laterals may substantially modulate the root system-rhizosphere interaction.

Folates in Plants: Research Advances and Progress in Crop Biofortification

Folates, also known as B9 vitamins, serve as donors and acceptors in one-carbon (C1) transfer reactions. The latter are involved in synthesis of many important biomolecules, such as amino acids, nucleic acids and vitamin B5. Folates also play a central role in the methyl cycle that provides one-carbon groups for methylation reactions. The important functions fulfilled by folates make them essential in all living organisms. Plants, being able to synthesize folates de novo, serve as an excellent dietary source of folates for animals that lack the respective biosynthetic pathway. Unfortunately, the most important staple crops such as rice, potato and maize are rather poor sources of folates. Insufficient folate consumption is known to cause severe developmental disorders in humans. Two approaches are employed to fight folate deficiency: pharmacological supplementation in the form of folate pills and biofortification of staple crops. As the former approach is considered rather costly for the major part of the world population, biofortification of staple crops is viewed as a decent alternative in the struggle against folate deficiency. Therefore, strategies, challenges and recent progress of folate enhancement in plants will be addressed in this review. Apart from the ever-growing need for the enhancement of nutritional quality of crops, the world population faces climate change catastrophes or environmental stresses, such as elevated temperatures, drought, salinity that severely affect growth and productivity of crops. Due to immense diversity of their biochemical functions, folates take part in virtually every aspect of plant physiology. Any disturbance to the plant folate metabolism leads to severe growth inhibition and, as a consequence, to a lower productivity. Whereas today's knowledge of folate biochemistry can be considered very profound, evidence on the physiological roles of folates in plants only starts to emerge. In the current review we will discuss the implication of folates in various aspects of plant physiology and development.

Natural variation in folate levels among tomato (Solanum lycopersicum) accessions

Folate content was estimated in tomato (Solanum lycopersicum) accessions using microbiological assay (MA) and by LC-MS. The MA revealed that in red-ripe fruits folate levels ranged from 4 to 60μg/100g fresh weight. The LC-MS estimation of red-ripe fruits detected three folate forms, 5-CH3-THF, 5-CHO-THF, 5,10-CH(+)THF and folate levels ranged from 14 to 46μg/100g fresh weight. In mature green and red ripe fruit, 5-CH3-THF was the most abundant folate form. Comparison of LC-MS with MA revealed that MA inaccurately estimates folate levels. The accumulation of folate forms and their distribution varied among accessions. The single nucleotide polymorphism was examined in the key genes of the folate pathway to understand its linkage with folate levels. Despite the significant variation in folate levels among tomato accessions, little polymorphism was found in folate biosynthesis genes. Our results indicate that variation in folate level is governed by a more complex regulation at cellular homeostasis level.

Natures balancing act: examining biosynthesis de novo, recycling and processing damaged vitamin B metabolites

Plants use B vitamin compounds as cofactors for metabolism. Biosynthesis de novo of these metabolites in plants is almost fully elucidated. However, salvaging of precursors as well as cofactor derivatives is only being unraveled. Furthermore, processing of these compounds when damaged by cellular activities to prevent deleterious effects on metabolism is emerging. Recent investigations indicate that the role of B vitamins goes beyond metabolism and are being linked with epigenetic traits, specific developmental cues, the circadian clock, as well as abiotic and biotic stress responses. More in depth investigations on the regulation of the provision of these compounds through biosynthesis de novo, salvage and transport is suggesting that plants may share the cost of this load by division of labor.

Cell cycle entry, maintenance, and exit during plant development

Growth and development of plants are driven by the continuous production of new cells at the meristems; hence, it is of pivotal importance for plants to precisely regulate the timing and extent of cell proliferation. Although over the past decades the molecular components underlying cell cycle progression have been the subject of intensive research, knowledge remains scarce on how the various elements connect with developmental pathways. Recently, advances have been made that link cell cycle entry with nutrient availability, cell division maintenance with stem cell organization, and cell cycle exit with reactive oxygen species and developmental programs.

Loss of function of folylpolyglutamate synthetase 1 reduces lignin content and improves cell wall digestibility in Arabidopsis

BACKGROUND

One-carbon (C1) metabolism is important for synthesizing a range of biologically important compounds that are essential for life. In plants, the C1 pathway is crucial for the synthesis of a large number of secondary metabolites, including lignin. Tetrahydrofolate and its derivatives, collectively referred to as folates, are crucial co-factors for C1 metabolic pathway enzymes. Given the link between the C1 and phenylpropanoid pathways, we evaluated whether folylpolyglutamate synthetase (FPGS), an enzyme that catalyzes the addition of a glutamate tail to folates to form folylpolyglutamates, can be a viable target for reducing cell wall recalcitrance in plants.

RESULTS

Consistent with its role in lignocellulosic formation, FPGS1 was preferentially expressed in vascular tissues. Total lignin was low in fpgs1 plants leading to higher saccharification efficiency of the mutant. The decrease in total lignin in fpgs1 was mainly due to lower guaiacyl (G) lignin levels. Glycome profiling revealed subtle alterations in the cell walls of fpgs1. Further analyses of hemicellulosic polysaccharides by NMR showed that the degree of methylation of 4-O-methyl glucuronoxylan was reduced in the fpgs1 mutant. Microarray analysis and real-time qRT-PCR revealed that transcripts of a number of genes in the C1 and lignin pathways had altered expression in fpgs1 mutants. Consistent with the transcript changes of C1-related genes, a significant reduction in S-adenosyl-l-methionine content was detected in the fpgs1 mutant. The modified expression of the various methyltransferases and lignin-related genes indicate possible feedback regulation of C1 pathway-mediated lignin biosynthesis.

CONCLUSIONS

Our observations provide genetic and biochemical support for the importance of folylpolyglutamates in the lignocellulosic pathway and reinforces previous observations that targeting a single FPGS isoform for down-regulation leads to reduced lignin in plants. Because fpgs1 mutants had no dramatic defects in above ground biomass, selective down-regulation of individual components of C1 metabolism is an approach that should be explored further for the improvement of lignocellulosic feedstocks.